Estimation Method of Elastic-Plastic Fatigue Crack Growth Rates under Variable Amplitude Loadings

Abstract

Tests to determine the growth of load-controlled fatigue cracks were carried out on two kinds of structural steels under constant-amplitude and repeated two- and three-step loadings in the postyield region. Crack growth increment and crack closure were measured by the computer-aided unloading elastic compliance method. A fracture mechanics parameter, ΔJ/(1-J max /C), was found to be a good parameter for expressing the crack growth rates under constant-amplitude loadings irrespective of the stress ratios, where ΔJ, J max and C are the J integral range, the maximum J integral and a constant related to apparent fracture toughness of the specimen used, respectively. One-directional cyclic deformation was observed to occur under load cycles with high mean stresses. Moreover, it was found that crack closure and plastic deformation under variable amplitude loadings were affected by load variation patterns and load sequences. However, fatigue crack growth rates under varying loadings where both one-directional and cyclic plastic deformation were observed to be significant could be well estimated by the linear summation rule of crack growth using ΔJ * /(1-J max /C), where ΔJ * is the J integral range-pair taking account of the above mentioned one-directional deformation.